From cppreference.com
voidresize(size_typecount); (1)(constexpr since C++20)voidresize(size_typecount,constvalue_type&value); (2)(constexpr since C++20)Resizes the container to contain count elements, does nothing if count==size().
If the current size is greater than count, the container is reduced to its first count elements.
If the current size is less than count, then:
2) Additional copies of value are appended.
Parameters
count - new size of the container value - the value to initialize the new elements with Type requirements -T must meet the requirements of
and
in order to use overload (1). -T must meet the requirements of
in order to use overload (2). Complexity
Linear in the difference between the current size and count. Additional complexity possible due to reallocation if capacity is less than count.
Exceptions
If an exception is thrown for any reason, these functions have no effect (
strong exception safety guarantee
). Although not explicitly specified,
is thrown if the capacity required by the new vector would exceed
.
In overload (
), if T's move constructor is not noexcept and T is not
into *this, vector will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.
(since C++11)Notes
If value-initialization in overload (
) is undesirable, for example, if the elements are of non-class type and zeroing out is not needed, it can be avoided by providing a
.
Vector capacity is never reduced when resizing to smaller size because that would invalidate all iterators, while the specification only invalidates the iterators to/after the erased elements.
Example
Run this code
#include<vector>#include<iostream>voidprint(autorem,conststd::vector<int>&c){for(std::cout<<rem;constintel:c)std::cout<<el<<' ';std::cout<<'\n';}intmain(){std::vector<int>c={1,2,3};print("The vector holds: ",c);c.resize(5);print("After resize up to 5: ",c);c.resize(2);print("After resize down to 2: ",c);c.resize(6,4);print("After resize up to 6 (initializer = 4): ",c);}Output:
The vector holds: 1 2 3 After resize up to 5: 1 2 3 0 0 After resize down to 2: 1 2 After resize up to 6 (initializer = 4): 1 2 4 4 4 4 Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 resize() passed value by value passes by const reference
C++98 the behavior of resize(size()) was not specified specified
C++11 1. elements were removed by using
2. the type requirements of T were incorrect 1. uses
2. corrected
C++11 overload (
) did not have the exception
safety guarantee of overload (
)added
C++11 elements were removed by using
due to the resolution of LWG 2033 does not specify the method
of removing elements
can remove elements in the middle of a vector, so the value type is required to be
so that the elements following the removed section can be moved forward to fill the gap. However, resize() can only remove elements at the end of the vector, making the
unnecessary.
Removing elements by using
implies that the elements are required to be removed from back to front.
See also
returns the maximum possible number of elements
(public member function)
returns the number of elements
(public member function)
returns the number of elements that can be held in currently allocated storage
(public member function)
checks whether the container is empty
(public member function)